It is the error completely depending on your application.Are you able to debug the issue?Can you share the code that handling the request on your server. So that we can identify the issue.By NathanDirection is important than speed

While end user send a request for execution to IIS, it's follows some rules and regulations. In some cases the request won't satisfied those rules and regulations in that case we may face this type of issues like HTTP 400 error.

If you want to know the more details about this error, I suggest you to go through the below link, may be this might be helpful to you.

Error 400 is a 'Bad Request' error, when you got this error ? if it occur at the time of file upload then you need to define the file upload limit in web.configmay be following tag will help you <system.web> <httpRuntime executionTimeout="110" maxRequestLength="20000" /> </system.web>ORThis error is some what related to security, if your URL path is more than 255 char then also this error may occurif you use some special char in URL then also it will throw the error, ( <,>,*,%,&,:,\,?. )look at below link for more detailshttp://www.iis.net/learn/troubleshoot/diagnosing-http-errors/troubleshooting-http-400-errors-in-iisThanksKoolprasd2003Editor, DotNetSpider MVMMicrosoft MVP 2014 [ASP.NET/IIS]